Alzheimer disease is one of dementia and is associated with declined cognitive function and change in personality as principal symptoms. With the progress of aging population, the number of Alzheimer disease patients keeps increasing. It is expected that the number of patients in Japan, the United States and Europe will become 7.3 million in 2014 from 5.6 million in 2004. Therefore, early development of a medicament for prophylaxis and treatment of Alzheimer disease is earnestly desired.
Alzheimer disease's pathological indications include three features of atrophy and/or fall-off of neurons, formation of senile plaques due to aggregation and/or deposition of Aβ and neurofibrillary changes due to abnormal tau proteins. Alzheimer disease is classified into major two groups, i.e. familial Alzheimer disease and solitary Alzheimer disease. For the former, causative genetic mutation has been identified and its phenotype was found to elucidate that increase in production of amyloid β peptide, especially Aβ1-42 consisting of 42 amino acid residues, in the brain is a major cause of the disease. However, a ratio of patients diagnosed to suffer from familial Alzheimer disease in a strict sense is 1% or less among the total patients. It is for solitary Alzheimer disease that accounts for 99% of patients that the most prompt elucidation of cause of the disease is desired. It is thought that solitary Alzheimer disease is caused by duplication of unidentified genetic risk factors, the presence of recessive gene, the presence of environmental risk factors, and the like. At present, based on the fact that genetic mutations in familial Alzheimer disease are common in ultimate increase in highly aggregating Aβ42, it is thought that the major cause of solitary Alzheimer disease, as undergoing similar pathological development to that of familial Alzheimer disease, may be increase in Aβ42. This idea is called a hypothesis of myeloid and at present investigation for the treatment is under progress based on this hypothesis.
Aβ is produced from its precursor protein via cleavage with a membrane-bound aspartate protease. β-Secretase cleaves the N-terminal of Aβ whereas γ-secretases cleaves the C-terminal thereof. A cleaved Aβ may after a while secreted out of the cells depending upon the synaptic activity. Aβ rapidly auto-aggregates so that a monomer Aβ, via a dimer, trimer and a soluble oligomer, forms protofibril, a prefibrous structure, which then forms and accumulates insoluble amyloid fiber. It has now become a major idea that negative effects of aggregated soluble Aβ oligomer to the nerve play an important role in pathological conditions of Alzheimer disease. A number of forms of soluble Aβ oligomer that may block neurotransmission have been reported, including a dimer, a trimer, an amylospheroid (53 kDa aggregate), Aβ56 (56 kDa aggregate), aggregates of up to 40 amino acid length, and the like.
At present, approach for therapy through clearance of Aβ, which may play a highly important role in mechanism for onset of Alzheimer disease, is widely done, among which is a study of an antibody therapy with an antibody to Aβ. It is thought that action mechanism of Aβ clearance by an anti-Aβ antibody includes phagocytosis by microglia via Fc receptor of an antibody, accelerated dissolution or suppressed aggregation of fibrous Aβ and binding of the antibody to Aβ in blood to accelerate exclusion of soluble Aβ form the brain. Two approaches are proposed, i.e. active immunity where an anti-Aβ antibody is induced by a vaccine and passive immunity where an anti-Aβ antibody per se is administered.
For the former approach, vaccine AN1792 with the use of Aβ per se as an antigen proceeded to phase II clinical test but the test discontinued since 6% patients suffered from cerebrospinal meningitis during the test. However, difference in part of high-order function was observed between patients who had an increased antibody titer and patients who had not (Non-patent reference 1). Besides, the brain of patient died during the clinical test was examined to reveal that senile plaque disappeared in the neocortex. Moreover, MRI before and after administration showed that the administration group exhibited a less volume of the brain than that of the placebo group. As such, although the clinical test discontinued due to adverse side effects, it was also proved that a vaccine with the use of Aβ per se as an antigen was efficacious. In future, development of a safer vaccine is desired.
It is thought that cerebrospinal meningitis is caused by AN1792 as a consequence of the use of a potent adjuvant QS21 and cellular immunity induced by a T cell epitope present in Aβ sequence per se. Plenty of investigation has been done for a T cell epitope present in Aβ sequence per se to reveal that, at least in the N-terminal of Aβ sequence of residues 1-10, a T cell epitope that may induce humoral immunity is present but a T cell epitope that may induce cellular immunity is not present (Patent references 2-4).    Non-patent reference 1: Gilman S, Koller M, Black R S, Jenkins L, et al., NEUROLOGY, 2005; 64: p 1553-1562    Non-patent reference 2: Michael G, Agadjanyan M G, et al., Journal of Immunology, 2005; 174: p 1580-1586    Non-patent reference 3: Bard F, Barbour R, Cannon C, Carretto R, Fox M, et al., Proc. Natl. Acad. Sci. USA, 2003; 100: p 2023-2028    Non-patent reference 4: Wanga C Y. et al., Vaccine, 2007; 25: p 3041-3052